Electric power utilities or transmission system operators own and operate electric power transmission networks interconnecting sites, such as power sources and substations, which despite being distant from each other some 100 km or more, have to be coordinated in one way or the other. Across their utility communication systems, a variety of messages can be transferred between distant sites of the utility over long distance communication links in order to safely transmit and distribute electric energy. For some of these messages, and in particular for teleprotection commands, the transmission delay between transmitter and receiver can be critical and should not exceed a few milliseconds up to some 10 ms.
Dedicated remote tripping devices or protection signal transmission devices, also known as teleprotection devices, can be used for transmitting protection or switching commands for distance and differential protection schemes in electrical high-voltage and medium-voltage networks and systems. Protection commands result, for example, in a circuit breaker being opened directly or indirectly and, in consequence in electrical disconnection of a selected part of the network or of the system. Conversely, other protection commands result in the opening of a circuit breaker in the remote station being prevented or blocked. In order for a protection command to be transmitted from one point of a power transmission or distribution network to another, a transmitter in a remote tripping device produces signals in accordance with the protection command, which are transmitted via a physical signal link. A receiver in another remote tripping device detects the transmitted signals and determines the corresponding number and nature of the protection commands. The physical signal link can involve radio waves or fiber optics, however generally, the protection signals are transmitted over pilot wires, analog leased lines, voice channels of analog or digital communication systems, or even high-voltage electricity transmission lines, the latter being known as power line communication (PLC).
US 2003/081634 A1 is directed to conventional audio tone teleprotection via a dedicated audio telecommunication link between two substations. This technology includes time division multiplexed frames being transmitted continuously from a sender to a receiver in a deterministic manner. At the sender, a special framing pattern (pre-selected pattern of 8 bits) is inserted in the last timeslot of each frame, thus decreasing bandwidth available for operational data. If the known framing pattern is not detected repeatedly at the receiver override information signals are inserted into the de-framed data stream to prevent a noise signal from producing a false output state.
For transmitting messages over long distances from one site to the other, the utility may rely on public or proprietary communication networks with non-deterministic behaviour. In this implementation, a Wide-Area communication Network (WAN) designates a packet switched communication network interconnecting two sites of the utility, and including a number of IP networks with specific network elements such as routers, switches, repeaters and possibly optical transmission media at the physical layer. WANs are in general very reliable, however the network elements can cause irregular network delays, occasional bit errors and inherent link failures, which all contribute to a non-deterministic behaviour of the network. In packet switched networks with individual data packets carrying destination addresses, heavy load on a communication channel or a specific network element can lead to increased delay or packet loss, whereas link failure can cause delays due to reconfiguration of the routers.
For time-critical applications, increased delay or packet loss can result in a malfunction of a system. For an electric power utility, in the worst case, substantial damage to a substation can occur if a trip signal is delayed. WANs can also be a target of unlikely, but potentially harmful acts of intrusion including e.g., inserting intentionally wrong commands at one of the routers. As a consequence, any communication channel involving a WAN may be considered both non-deterministic, or non-synchronous, and non-secure. Use of non-deterministic communications for command and control means that one can not guarantee delivery nor the actual communication path taken by a packet. Specifically, the use of the Internet increases the risk of critical control system communications failure, as attacks against other entities could greatly impact any control communications that uses this path or shares resources that touch the Internet.
Known dedicated teleprotection systems monitor the state and delay of a communication system by means of dedicated loop test messages that operate as follows: two stations, A and B, are connected via a communication link. Station A transmits a special message to station B, which receives it and immediately sends back an “echo” to station A. When station A receives this “echo”, it knows that the communication link is working, and it can also measure the transmission delay (half the time it takes the loop test message to travel from A to B and back to A). A loop test message can be sent once every few hours. As a result, changes of the transmission delay in real-time can not be detected.
Alternatively, the delay measurement method specified in IEEE 1588 (IEEE Standard 1588-2002, IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems, (e.g., Precision Time Protocol PTP)) can be used to monitor a state and/or availability of a communication system. Standard two-way time synchronization protocols such as IEEE 1588 define methods for synchronising devices via a communication network such as a Local Area Network (LAN), to a higher precision (e.g., better than one microsecond).
In the field of Voice over IP (VoIP), voice calls are routed over an Internet Protocol (IP) network, and a Quality of Service (QoS) is an important issue between the service provider and the end user. In this context, and more generally for the purpose of real-time data transmission, the Real-Time Protocol (RTP) within the ISO-OSI layer reference model prescribes the encapsulation of e.g. encoded voice data in RTP packets. The latter are passed to the transport layer and further to the Internet Protocol (IP) network layer. At the transport layer, data transmission systems may use either a reliable protocol (such as a Transmission Control Protocol TCP) or an unreliable protocol (such as User Datagram Protocol UDP). The former ensures that all the packets arrive at the receiver, but specifies more bandwidth due to protocol overhead and it introduces more delay. The reliable transport protocols normally measure the round-trip delay in order to derive there from when messages should be repeated. On the other hand, unreliable protocols are lightweight and faster although the data stream can be subject to packet loss.
In US 2007/0230361 A1, a method is provided for monitoring a packet-switched network via which real time VoIP data is transmitted. Data packets containing real-time data are sniffed in order to monitor a QoS parameter. The QoS parameter includes one of egress delay, ingress delay, jitter, roundtrip delay, packet loss, throughput, instantaneous signal loss, and accumulated content loss. In another patent application US 2002/105909 related to VoIP, as long as the smoothing algorithm, which adjusts for transitory effects while evaluating packet loss data, yields acceptable values, calls continue to be routed over the IP network. If, on the other hand, the value exceeds a threshold, a QoS Monitor blocks routing over the IP network and routes calls over an alternative network, such as a Switched Circuit Network (SCN).